The present invention relates to a drive unit with a planetary gear system and a hydrodynamic retarder. The retarder is used for power transmission during acceleration at low speed and during deceleration.
Smooth starting and stopping of a vehicle are enhanced by fluid coupling between input and output. However, the efficiency of fluid coupling is relatively low. It is thus desirable to switch out or inactivate the fluid coupling when running speed is achieved. One type of fluid coupling is a retarder which is a type of hydrodynamic brake.
German laid-open Patent Specification No. 1,480,506 discloses an apparatus for driving a zero-shaft in the steering system of a tracked vehicle. This apparatus includes planetary gears and a retarder. The retarder can be connected into the planetary gear system to decelerate one of the gear elements which rotates faster than an input gear element and thereby to couple torque to an output in a direction opposite to that of the first-mentioned gear element.
German laid-open Patent Specification No. 2,021,543 discloses a vehicle gear system including an integrated hydrodynamic torque converter which can be used for traction, particularly for starting, and for hydrodynamic braking. Although this system operates satisfactorily, it requires a large number of planetary wheel sets to accomplish a given number of gear ratios.
German laid-open Patent Specification No. 2,656,669 discloses a vehicle gear system having both a hydrodynamic torque converter, intended purely for traction operation, and a retarder, which is used only for hydrodynamic braking. The need for both types of apparatus makes the cost of such a gear relatively high.
Both of the above gear systems incur hydraulic losses in the hydrodynamic power transmission especially during startup.
German Offenlegenschrift No. 2,935,361 discloses a gear system which has a mechanical, so-called continuous and gear-switching brake, which is designed for longer periods of slip operation. Slip operation is provided similarly to a retarder for starting up from a standstill and for braking the vehicle. Despite the fact that the brake is constructed for longer periods of slip operation, the danger of wear and/or overheating remains. It is, therefore, not used in practice. Moreover, it is not acceptable to replace the continuous brake in the known gear system by a retarder, since coupling the continuous braking rotor to a slowly-rotating gear component provided therein would be very unfavorable for the retarder.
U.S. Pat. No. 2,343,509 discloses a gear system which has a retarder, a planetary gear system and a through-coupling. The through-coupling is a free-wheeling one-way coupling. When initially starting from a standstill, the input shaft, the output shaft and the retarder rotor have the same rotary speed and the same direction of rotation. Thus, the retarder can become effective only after reaching a certain minimum rotary speed. The output shaft then rotates faster than the input shaft. The retarder in this patent is not useful in the important start-up speed range. Retarder braking is not disclosed.
German patent No. 2,618,073 discloses an electrohydrodynamic drive unit which has a torque converter and a reversing gear for traction operation.
German patent No. 1,600,191 and German published Patent Specification No. 3,000,664 disclose retarders adapted for bi-directional rotation with high specific output in both directions.